1. Field of the Invention
This invention relates to an automatic film winder for camera, and more particularly it relates to the improvements in the automatic film winder of the type in which the film end is wound up on a spool by dint of a frictional force alone.
2. Description of the Prior Art
Among the known automatic film winders of this type is a device in which when loading a film cartridge in the camera by opening its back cover, the film end (in most cases the narrowed leading end of the film) slightly projecting from the cartridge is placed on a sprocket and then, after closing the back cover, the ordinary film winding operation is performed to wind up the film on the spool. According to this system, the film with the perforations at the narrowed leading end thereof being engaged with the projections on one side of the sprocket is moved toward the spool side with the sprocket rotation effected by the film winding operation, and at a point where the narrowed leading end of the film has just contacted the external surface of the spool, a frictional force is produced between the film and the external surface of the spool to thereby wind up the end of the film on the spool.
The matters of primary concern for this type of automatic film winders, that is, the shape and size of the narrowed leading end of the commercial films, particularly 35 mm perforated films, film withdrawal load, etc., vary slightly from the products of one maker to those of the other maker, and it was found as a result of experiments that such variance gives a delicate influence to the automatic film winding performance, in other words, the condition of winding of the film.
Regarding the condition of winding of the film, it is required that when the narrowed film end is wound up on the spool, the film is delivered in such a manner that the edge thereof will advance on a linear track decided by the rail position at the exposure aperture and the position of the sprocket projections (such linear track being hereinafter referred to as ideal linear track) and the end face of the film laminate on the spool will be always positioned on said ideal linear track regardless of the volume of film wound up, and that the film is wound up closely on the peripheral surface of the spool. This is not merely a matter required for the automatic film winding mechanism alone; it is a general principle applied to the film winding on a spool. However, when the narrowed leading end of the film is actually fed to the spool side with the sprocket rotation, it is often experienced that the film end tends to stray from said ideal linear track, that is, when the film is loaded in the camera, the film is fed slightly aslant upwardly in case the narrowed leading end is formed on the lower side of the film as usual.
When this occurs, there is induced a phenomenon of "loose winding", that is, when the narrowed film end has just made a round of the spool, the tip of said film end bites into the succeeding perforation and a loop with a greater diameter than the spool is formed. There may also take place a so-called "bamboo shoot phenomenon" in which the narrowed leading end of the film is coiled spirally on the spool just like the surface of a bamboo shoot.
"Loose winding" leads to serious troubles, that is, when the film is wound up to a certain roll diameter, the external surface of the film roll presses against the inner wall of the film chamber to make any further winding impossible. Also, rewinding of the film becomes awkward or "heavy" to perform, and this may result in severance of the film. The "bamboo shoot phenomenon" causes the film being wound up to be forcibly biased to one side to produce a "local stretched state". This may not only make any further film winding impossible but may also cause break of the perforations in the film. Also, rewinding of the film is resisted to cause cutting of the film in the worst case.
Methods have been proposed for preventing such phenomena. For example, Japanese Utility Model Application Publication No. 41,378/1977 shows a method according to which a taper is formed on the inner wall surface of the film winding chamber and a friction member is rotated along such taper wall surface to thereby attain the intended object.
Such method, however, is unsuited for adaptation to the cameras of the latest models which have an advanced extent of compactness and lightweight. For example, formation of a taper on the inner wall surface of the film winding chamber means formation of a thick-walled portion and a thin-walled portion of the wall of the film winding chamber, and since the gradient required for preventing said loose winding and bamboo shoot phenomenon may reach close to about 3 degrees, the difference between said thin-walled portion and thick-walled portion is not small. This poses a problem on relation of mechanical strength between said thick-walled and thin-walled portions when designing a camera with maximum compactness and lightweight. If the camera is designed to provide sufficient strength to the thin-walled portion, then overmuch strength will be given to the thick-walled portion, while if designing is made by giving preferential consideration to the strength of the thick-walled portion, then the thin-walled portion may become insufficient in strength. This problem is conspicuous when the camera body is made from plastic molding. Also, such problem is not limited to the film winding chamber; it also arises when a tapered thin-walled cylindrical member or members are used in the film winding chamber.
On the other hand, for adapting a friction member so as to be rotatable along the taper surface, it is required to form the inner wall surface of the film winding chamber cylindrically or to make an arrangement such as to allow displacement of the friction member in the radial direction. This greatly limits the degree of freedom in designing of a compact and lightweight camera.
In order to solve these problems, the present inventors have previously proposed a device featuring formation of a convexity or preparation of the protrusion member at a part of the inner wall surface of the film winding chamber, whereby the plane of the narrowed leading end of the film is slanted relative to the axis of the film winding member by making use of the level difference between said convexity and the inner wall surface so that the advancing direction of the narrowed film end will be thereafter oriented aslant downwardly. According to this device, however, for the reasons that said convexity is in many cases formed close to the film receiving position of the film winding chamber and that the narrowed leading end of the film is always given an upward force by the projections on one side of the sprocket, there is a possibility that the advancing direction of the film be changed upwardly in the course of advancement of the film end till it reaches close to said film receiving position after making a round of the film winding member. Therefore, it is required first of all to eliminate such possibility.
Since said regulation on the advancing direction of the narrowed leading end of the film in the course of its movement round the film winding member is attained by slanting the plane of the narrowed leading film end relative to the axis of the film winding member as said before, it is matter-of-factly desired to effectuate said film plane slanting more positively and surely.